# std::ranges::construct_at

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< cpp‎ | memory

Defined in header `<memory>` |  |
Call signature |  |
template< class T, class... Args > constexpr T* construct_at( T* location, Args&&... args ); |  | (since C++20)

Creates a `T` object initialized with the arguments in args at given address location.

Equivalent to if constexpr (std::is_array_v<T>)
`` return ::new (`voidify`﻿(*location)) T[1]();
else
`` return ::new (`voidify`﻿(*location)) T(std::forward<Args>(args)...); , except that `construct_at` may be used in evaluation of constant expressions(until C++26).

When `construct_at` is called in the evaluation of some constant expression expr, location must point to either a storage obtained by std::allocator<T>::allocate or an object whose lifetime began within the evaluation of expr.

This overload participates in overload resolution only if all following conditions are satisfied:

- std::is_unbounded_array_v<T> is false.
- ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed when treated as an unevaluated operand.

If std::is_array_v<T> is true and sizeof...(Args) is nonzero, the program is ill-formed.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

location | - | pointer to the uninitialized storage on which a `T` object will be constructed
args... | - | arguments used for initialization

### Return value

location

### Notes

`std::ranges::construct_at` behaves exactly same as std::construct_at, except that it is invisible to argument-dependent lookup.

### Example

Run this code

    #include <iostream>
    #include <memory>

    struct S
    {
        int x;
        float y;
        double z;

        S(int x, float y, double z) : x{x}, y{y}, z{z} { std::cout << "S::S();\n"; }

        ~S() { std::cout << "S::~S();\n"; }

        void print() const
        {
            std::cout << "S { x=" << x << "; y=" << y << "; z=" << z << "; };\n";
        }
    };

    int main()
    {
        alignas(S) unsigned char buf[sizeof(S)];

        S* ptr = std::ranges::construct_at(reinterpret_cast<S*>(buf), 42, 2.71828f, 3.1415);
        ptr->print();

        std::ranges::destroy_at(ptr);
    }

Output:

    S::S();
    S { x=42; y=2.71828; z=3.1415; };
    S::~S();

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3436 | C++20 | `construct_at` could not create objects of array types | can value-initialize bounded arrays
LWG 3870 | C++20 | `construct_at` could create objects of cv-qualified types | only cv-unqualified types are permitted

### See also

ranges::destroy_at (C++20) | destroys an object at a given address (algorithm function object)
construct_at (C++20) | creates an object at a given address (function template)

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